This invention pertains to pulverizing mills, and more particularly to the plunger can structures which contain mechanical spring suspension systems used in such mills, and to spring compressor systems which easily and safely open and close such plunger cans.
Pulverizing mills are used to pulverize coal, limestone and other solid materials. In the case of coal, gravel sized coal enters the mill and is pulverized into a powder. The powder is carried out of the pulverizer by a high velocity air stream and into a furnace where it explosively burns to heat steam which, in an electrical power generator, drives a turbine to generate electricity. The pulverizers are designed to operate continuously, except during periods of repair. Examples of these kinds of coal pulverizers are in U.S. Pat. No. 4,705,223 by Dibowski et al.; U.S. Pat. No. 4,694,994 by Henne et al.; U.S. Pat. No. 4,679,739 by Hashimoto et al.; U.S. Pat. No. 4,522,343 by Williams; U.S. Pat. No. 4,491,280 by Bacharach; and U.S. Pat. No. 4,717,082 by Guido et al.
The pulverizing is accomplished by directing the coal onto grinding tables which interface with pulverizing rollers. The rollers are each mounted on a separate roller assembly shaft, and each roller assembly shaft is mounted on a clamshell door in the pulverizer. Typically, the grinding table is a disc-shaped member with an annular groove or raised circumferential edge in the top surface. The grinding table rotates so that the annular groove mates with the rollers. The coal is introduced from the top of the assembly and feeds by gravity to the annular groove where it is pulverized as the grinding table rotates under the rollers. The pulverized coal dust is discharged from the grinding table by a high velocity air flow deflected over the grinding table. The coal dust is redirected through and out of the pulverizing mill by subsequent deflection of the combined flow of air and suspended coal dust particles.
The pulverizing mill may use a rotating grinding table with stationary roller assemblies, as described in U.S. Pat. No. 4,717,082 by Guido et al. (the contents of which are hereby incorporated by reference), and additional examples of these kinds of roller assemblies are in U.S. patent application Nos. 07/464,870 filed Jan. 16, 1990 by Parham and U.S. Pat. No. 07/539,574 filed Jun. 18, 1990 by Parham. Alternatively, the pulverizing mill may use a stationary grinding table and several rotating roller assemblies. The roller assemblies may also be independently biased against the grinding table so that vibration and shock on one roller will not be transferred to all the other rollers, as described in the Guido patent. The rollers and grinding table are massive; each roller weighs several tons and is on the order of five feet in diameter.
The roller assemblies are biased towards the grinding table by means of compression spring assemblies. Because of the large size of present pulverizing mills and grinding rollers, compression spring assemblies exerting forces within the range of 25,000 to 30,000 PSI are common. Those compression spring assemblies typically are housed in a plunger can structure (sometimes referred to in the art also as a "Journal Spring Housing" or "Spring Housing" as a constituent part of a "Mechanical Spring System") which is suitably mounted so as to cooperate with the roller assembly. A typical plunger can structure houses several elements, including a compression spring assembly, a plunger assembly which transfers the force generated by the compression spring to the roller element of the roller assembly, and a plunger bearing assembly, all of which are well known in the art (the plunger assembly is sometimes referred to in the art as a "Stud Assembly" or "Preload Stud Assembly"). Examples of these kinds of plunger can structures and the assemblies housed therein are in U.S. Pat. Nos. 3,881,348 by Morton, U.S. Pat. No. 4,706,900 by Prairie, et al. and U.S. Pat. No. 4,759,509 by Prairie.
The plunger can structure currently in standard use is a fabricated can comprising a multiplicity of parts. Each of the assemblies housed within the can also comprises a multiplicity of parts. Between the plunger can and the assemblies housed within it, and among the several assemblies, are bushings, bearings and other fittings by which the interfacing elements of each assembly suitably come into contact with, or mutually support and hold in place, interfacing elements of the other assemblies and the plunger can structure. These interfacing elements comprise a further multiplicity of parts.
The plunger can structure itself as well as the compression spring assembly, the plunger assembly, the plunger bearing assembly, and all of the interfacing and other elements of each assembly contained within the plunger can are exposed to extreme conditions. The massive roller assemblies with which they cooperate typically revolve at 200 to 300 revolutions per minute. The pulverizing mills within which many of the plunger cans are installed operate at a temperature around 600 to 700 degrees F. In addition, the mills occasionally catch fire. Such fires are frequently smothered with steam and then cooled, resulting in large and fast temperature changes in the pulverizing mills. There is also the constant presence of pulverized coal dust particles throughout the pulverizing mills. Carried by high speed air flow, the coal particles in motion create the effect of a continuous sand-blasting on all component structures within the interior of the pulverizing mill.
The existing multi-part fabricated can, cooperating with its several multi-part assemblies and interfacing elements under the extreme conditions of the pulverizing mill, is a source of a number of costly problems. These problems affect both the fabricated plunger can structure and the assemblies it houses. One problem is that the fabricated plunger can wears out or one or more of the multiplicity of parts comprising it wears out. Such wear in the fabricated plunger can is a product of vibration, abrasion and shock, and is accentuated by differential shrinkage and expansion of its various elements in reaction to heating and cooling in the pulverizing mill. Stress cracks and fractures are not uncommon in the fabricated plunger can structure. So also, and by similar causes, the compression spring assembly, plunger assembly, plunger bearing assembly and interfacing elements contained within the fabricated plunger can structure experience structural degradation, deterioration misalignment and wear. Other degradation to the assemblies is caused by the cumulative blasting effect, deposit over time, and consequent caking of, coal dust particles around the elements of such assemblies.
Repairing the existing fabricated plunger can structures themselves, and opening them so as to inspect, clean, adjust, or repair or replace the compression spring assembly, plunger assembly, plunger bearing assembly and interfacing elements contained within them presents other difficulties. The compression spring in the plunger can may be under twenty thousand pounds or more of pressure, so that the top tends to explode off the can like a bomb when it is removed, thereby endangering the workmen and surroundings. Also, the existing fabricated plunger can structures must be removed from the pulverizing mill for opening off site. This requires labor and takes time. The pulverizing mill cannot operate during that time, and the down time imposes a cost of many thousands of dollars per day. Electric utilities seek to pass that cost on to rate payers or else absorb it so as to suffer diminished rates of return to their shareholders.
Moreover, wear and degradation to the plunger can structure and to the assemblies housed within it adversely affect the massive roller assemblies of the pulverizing mill. In particular, the wear rate of the roller assemblies is sensitive, not only to the depth, hardness and uniform size and consistency of the coal, but also to the amount and uniformity of the countervailing force applied to the rollers by the compression spring and other assemblies housed within the plunger can structure. The cost of repairing or replacing the rollers is very high in relation to the cost of repairing or replacing the plunger can structures and any of the assemblies contained therein.
Plunger can structures may be mounted so as to be located almost completely externally to the interior chamber of the pulverizing mill, as shown in the Prairie patents referenced previously, including U.S. Pat. No. 4,706,900 by Prairie et al. Although can structures so mounted may thereby lessen the exposure of the plunger can to the extreme conditions existing within the mill, they are inherently more dangerous to the workmen and surroundings because they lack the protective shielding of the mill as in an internally mounted plunger can. Also, none of the existing devices described in the patents recited above or elsewhere adequately addresses the problem of improving the plunger can structure itself so as to better withstand those extreme conditions. Further, none of the existing devices described in the patents recited above or elsewhere adequately addresses the problem of providing a spring compressor system that easily and safely opens the plunger can in place to permit the inspection, repair, adjustment or replacement of the compression spring assembly, plunger assembly, plunger bearing assembly and interfacing elements contained within the plunger can. Finally, none of the existing devices described in the patents above or elsewhere allow the plunger can to be adapted specially to the unique operating characteristics of the pulverizing mill in which it will be mounted.